As for superconducting wires using oxide superconducting materials, there are the following two kinds that are under intensive development at present: one is a tape-shaped silver sheathed superconducting wire made by a powder-in-tube method and having (Bi,Pb)2Sr2Ca2Cu3O10±δ phase as a main component (δ is a number on the order of 0.1: hereinafter, referred to as (Bi,Pb)2223). (For example, refer to Non-patent Document 1.) The other one is a tape-shaped thin-film superconducting wire in which a superconducting layer is formed on a metallic substrate by a vapor-phase method or a liquid phase method. The superconducting material of the thin-film superconducting wire is an oxide superconducting material represented by the chemical formula of RE1Ba2Cu3Ox (x is a number which is near 7; hereinafter, referred to as RE123), and in the RE (Rare Earth) part one element or compound of rare-earth elements such as Y, Ho, Nd, Sm, Dy, Eu, La, Tm, etc. is arranged. (For example, refer to Non-patent Document 2.)
A superconducting coil is produced using the above-mentioned superconducting wires for the purpose of a magnetic field application. Patent Document 1 discloses a superconducting coil that is made by stacking a plurality of pancake coils using tape-shaped (Bi, Pb)2223 superconducting wires. The superconducting coil made of the tape-shaped (Bi,Pb)2223 superconducting wires is cooled to a low temperature of 20 K or less, and a magnetic field is generated by flowing a given operating current.
The tape-shaped (Bi,Pb)2223 superconducting wire is not so strong in terms of resistance to a magnetic field at high temperature, and the critical-current value tends to be degraded when the superconducting wire is placed in the magnetic field. Therefore, when the (Bi,Pb)2223 superconducting wire is in a shape of coil, the critical-current value decreases due to a magnetic field generated by itself. As a countermeasure, therefore, the critical-current value is made larger beforehand by lowering the operation temperature so that a sufficient electric current may flow through the coil under the generated magnetic field. Thus, if a comparatively large magnetic field is to be generated in a superconducting coil in which the tape-shaped (Bi,Pb)2223 superconducting wire is used, the coil is cooled to a low temperature of about 20 K. Therefore, for cooling the superconducting coil, it is necessary to use equipment that is capable of cooling to a low temperature of about 20 K.
On the other hand, the tape-shaped RE123 superconducting wire is superior to the tape-shaped (Bi,Pb)2223 superconducting wire in terms of the resistance to a magnetic field, and the degradation of the critical-current value is small under the relatively high temperature in the magnetic field. However, the tape-shaped RE123 superconducting wire, the manufacturing process of which is complicated and delicate, is difficult to make a long length of uniform wire that can be formed into a coil with a single length thereof. Also, the wire cost is high because of the low yield.
[Patent Document 1] Japanese Patent Application Publication No. H10-104911
[Non-patent Document 1] SEI Technical Review No. 169 issued in July 2006, pp. 103-108
[Non-patent Document 2] SEI Technical Review No. 169 issued in July 2006, pp. 109-112